In some current refinery operations, aqueous sodium hydroxide is used to scrub out sulfur from hydrogen sulfide and mercaptans in refinery streams generated by thermal cracking and/or hydrotreating-type refinery processes. The disposal of the resulting sodium sulfide solutions (spent caustic (NaOH)) is costly and environmentally detrimental. Current NaOH regeneration techniques are also costly. For example, Allied Signal's electro dialysis cell is expensive and requires the use of membranes. Some refineries abroad expend spent caustic by injecting it into deep wells. However, this is frowned upon in the United States. Thus, the spent NaOH is disposed of via more costly routes (e.g., burial).
Methods also exist for the regeneration of aqueous alkali metal hydroxide. See e.g., U.S. Pat. No. 4,163,043 discussing regeneration of aqueous solutions of Na, K and/or ammonium sulfide by contact with Cu oxide powder yielding precipitated sulfide which is separated and re-oxidized to copper oxide at elevated temperatures and an aqueous solution enriched in NaOH, KOH or NH.sub.3. Romanian patent RO-101296-A describes residual sodium sulfide removal wherein the sulfides are recovered by washing first with mineral acids (e.g., hydrochloric acid or sulfuric acid) and then with sodium hydroxide or carbonate to form sodium sulfide followed by a final purification comprising using iron turnings to give insoluble ferrous sulfide.
Future feeds are expected to be more sulfur laden, yet allowable sulfur levels, in such feeds, are expected to be more stringent due to increasing environmental constraints. Hence, the increasing amounts of the sodium hydroxide used for desulfurization of such feeds will require efficient, economical regeneration.